Verse 24 contains the unique phrase "after they had ended the ceremony…" It is unclear what this means. Perhaps there was some formal greeting. Perhaps there was a formal ceremony of surrender by the former Noah-men to the Gideon-men. Perhaps it was simply a word that described the exchange of information between the two groups. Another possibility is that Mormon is again conflating events. Perhaps the meeting of the two groups is occurring at approximately the same time as the exchange of oaths noted below, and it is that exchange of oaths that is the "ceremony." Gideon may have sent his men "secretly" while the rest of the city was occupied in the "ceremony" of formal submission to the Lamanites.
John Tvedtnes has suggested that the ceremony would have been a purification ceremony whereby the men were returned to ritual cleanliness after their murder of the king (Tvedtnes, John A. "The Nephite Purification Ceremony." In: The Most Correct Book. Cornerstone, 1999, pp. 176-7). This suggestion makes sense in terms of Jewish halakic (purity) laws, but does not necessarily fit the Biblically proscribed actions for a murderer. As Tvedtnes points out, the murderer would not be allowed to live under the law of Moses. While an accidental killing allowed for refuge, it would be hard to consider the death of Noah accidental. Nevertheless, there may have been some aspect of the Nephite culture (or at least this branch of Nephite culture) that provided for a ritual cleansing of those who had committed a similar act.